1. Field of the Invention
This invention relates to Charged Particle Beam Projection Systems employed in lithography, and more particularly to deflector coil structures employed in such systems and to methods of manufacture of such coil structures.
2. Description of Related Art
In Charged Particle Beam Systems used for lithography, such as E-beam (electron beam) lithography systems, electromagnetic deflectors are typically used to deflect an E-beam to a desired landing point where it exposes a substrate, e.g a mask, reticle, or wafer to the energy of the E-beam.
One type of deflector in common use is the toroidal deflector comprised of a number of discrete coils arranged radially about a central axis as described in U.S. Pat. No. 5,631,615 by Messick and Senesi for "Free Wound Electromagnetic Deflection Yoke". A deflector assembly of this type is typically referred to as a deflection yoke.
As the integrated circuit industry continues to shrink the requirements of the scale of critical dimensions and tolerances for across-chip-linewidth-variation, the mechanical stability and symmetry of the deflector in an E-beam exposure system becomes critical, if placement errors and optical aberrations are to be avoided.
This is especially true in the evolving field of E-beam projection lithography, often referred to by the acronym EBPS (E-Beam Projection Systems), where large deflection fields require application of large amounts of power to deflection yokes, leading to the dissipation large quantities of heat, which has led to thermal effects. As a result, thermal effects caused by current passing through the coils of deflection yokes cause dimensional changes of coil size and location within the deflector assembly which is a major contributor to the mechanical instability of existing deflection yoke designs. Mechanical changes to the coils result in changes to the magnetic deflection field, which in turn results in placement errors plus aberration errors.
Controlling coil temperatures during the operation of an E-beam exposure system for E-beam lithography is critical in maintaining mechanical stability. Temperature control must be maintained while the power dissipated in the coil varies constantly. To maintain a nearly constant temperature, deflectors often operate in a flow of a liquid coolant such as Fluorinert. This is only partially effective, even with the best coolants, because the coolant can reach only the surface of the coils, and has little influence on the core temperature within the tightly wrapped coils. The result is that the coils continue to expand and contract even when and if the surface temperature is being held constant.
In the prior art, conventionally wound strands of copper wire coils have been used in conjunction with liquid or air cooling, but the mechanical problems described above have not been overcome. This problem is well known in the field of E-beam lithography, yet no adequate solution has been forthcoming, heretofore.